The subject matter disclosed herein relates to a permanent magnet (PM) machine system, and more specifically to a PM machine system that controls a PM machine such that the PM machine does not substantially exceed a pullout torque value.
Wind turbines are one alternative to conventional sources of energy. The blades and hub of the wind turbine are referred to as a wind turbine rotor. The wind turbine rotor is typically connected to a permanent magnet (PM) generator through a gearbox. A wind turbine rotor that is rotating at very high speeds tends to obstruct wind flow, which in turn reduces power extraction of the wind turbine. For example, a strong gust of wind can potentially cause the wind turbine rotor to rotate at excessive speeds. The PM generator may be used to control the speed of the wind turbine rotor. Specifically, torque may be exerted by the PM generator to control the speed of the wind turbine rotor.
A PM generator typically includes a stator and a rotor, where the stator may include a three phase stator winding that forms a cylindrical cavity. The rotor of the PM generator rotates within the stator cavity by providing three phase electrical voltages to the stator windings. The stator voltages generate stator currents that create a rotating stator field. The stator field corresponds to a stator magnetomotive force (stator mmf). The torque produced by a PM generator is proportional to the sine of an angle located between the rotor angular position and the stator mmf. Specifically, the peak torque produced by the PM generator is typically referred to as pullout torque. If an attempt is made to increase the torque value past the pullout torque value by increasing the current, then less torque will actually be produced. This condition is commonly referred to as motor pullout and refers to a point where a machine operates past the pullout torque value.
The torque of the PM generator is also proportional to the magnitude of the stator flux. However, when connected to a power converter, the stator flux of the PM generator is limited by the peak converter output voltage. As the speed and frequency of the PM generator increases, the stator flux decreases. This may create an issue in the event that a strong gust of wind causes the wind turbine rotor to rotate at very high speeds. This is because as the wind turbine rotor approaches an over speed condition, a reduction in flux in the PM generator is needed to generate the torque required to control the speed of the wind turbine rotor. However, as the wind turbine rotor approaches the over speed condition, the speed and frequency of the PM generator are increased, therefore reducing the stator flux. Thus, the ability to exert torque by the PM generator to control the speed of the wind turbine rotor is diminished.